One of the most frightening experiences a person can have is to be inside a large building and learn that there is a fire. In order to give people an early notice and a chance to escape to the outside, it has been conventional to provide some type of an alarm when fire conditions exist. Many decades ago, such alarm was given merely by the first person observing the condition to shout "fire" and the shout would be taken up by others. As our society has become more complex and our technology has become more sophisticated, it has become conventional to provide a wide variety of detectors located in several positions which are capable of initiating audible and/or visual signals to warn people. As buildings become larger and especially high-rise, the detecting devices and alarm signals have become quite sophisticated.
In recent years, it has been considered desirable to provide both visual and audible alarm signals and legislation has been passed in some jurisdictions requiring both audible and visual alarms. This is considered desirable as deaf people cannot hear an audible alarm and blind people will not recognize a visual alarm. It has also been found that intermittent operation of the audio and visual signal is a superior means for gaining attention and response.
In large enclosed areas, such as a shopping mall, and/or in multi-story buildings, it is frequently necessary to provide many visual and audio signals. Although the amount of power consumed by each unit is not particularly significant designers of such equipment have recognized that they could use a smaller and more economical power supply if it were practical to control the maximum number of devices which were connected at any given instant. For example, the capacity of the power supply could be cut in half if it were possible to have the duty cycles of the alarms arranged so that while one half of them is activated, the other half is inactivated and vice versa. With the use of both visual and audible components, it was recognized that further savings could be made in the power supply system if the audio component was energized while the visual component was not and vice versa. It is fairly obvious that such a technique could be readily implemented by coupling the audio devices in parallel on one cable pair and the visual devices in parallel on another cable pair. However, it is equally obvious that this may result in a cost trade-off wherein the extra cable pair is as expensive as providing the larger capacity power supply.
The structure of the present invention is provided to permit the use of an economical power supply without requiring the use of an extra cable pair.
U.S. Pat. No. 3,431,470 issued Mar. 4, 1969 to Stiebel et al teaches a means for excercising accurate control of the time period during which an alarm is activated. With this accurate timing, one could devise a system wherein groups of alarms could be activated during non-overlaping time periods.
U.S. Pat. No. 3,456,251 issued July 15, 1969 to G. E. Smith et al teaches a plurality of intermittent visible and/or audible indication devices all controlled by a single circuit. However, the control is not exercised over a single pair of wires.
U.S. Pat. No. 3,478,345 issued Nov. 11, 1969 to Greenwald teaches a system using cam pins in slots on a rotating drum to control a timing sequence of audio visual signals. This system, if used to minimize maximum instantaneous power consumption, would require the use of multiple leads as aforedescribed.
U.S. Pat. No. 3,872,471 issued Mar. 18, 1975 to Bird et al discloses a bell with self contained timing means for intermittent actuation. If a plurality of these two terminal devices were used on a single cable pair, there would be no way to assure that they might not all be activated at any given instant, and therefore, power supply capable of providing sufficient energy to operate all of them would be required.